With
a TH-700R4 transmission installed in place of a TH-350 or
TH-400, what can be expected as a reasonable mileage increase?

Feedback from customers over the last 11 years, indicates
mileage improvements of 30% on the low end to over 70% in
a couple of well documented cases is about the range you
should expect from a conversion to one of these overdrives.
For example, if you now get consistently 12 miles per gallon,
expect to see 16 to 20 miles per gallon after the change
over. One of our favorite customers, had a 64 El Camino
with a stock 283 cubic inch small block with a Rochester
two barrel and a Powerglide transmission. He had kept a
meticulous log of his gas mileage since the original purchase
and over the years had gotten an average of 18.6 miles per
gallon. With no other changes, we just installed one of
our overdrive transmissions, his mileage jumped to 28.6
miles per gallon! He did continue to drive at the same 60
MPH while on the highway to achieve these figures. The engine
RPM will drop over 35 % when you install one of these transmissions.

If
your engine RPM drops from say 3000 RPM to approximately
2000 RPM you will typically start driving at a higher average
rate of speed on the highway because it will seem so natural
to do so. If you choose to cruise 10 to 15 miles per hour
faster on the highway than you did before the transmission
upgrade, you probably won't see the dramatic increase in
fuel economy number, but you certainly will get from Point
A to Point B a lot quicker, smoother and quieter! One thing
is certain, when our customers have given us feed back,
they invariably state they are getting "very noticeable"
improvements in Mileage.